Interfacial dilational elasticity

Noiiy Ring methods, pendant and spinning drop methods, for surface and interfacial dilational elasticity, thin-film techniques, and surface lateral diffusion when using fluorescence recovery after photobleaching (FRAP) methods. 

Rheology is the study of the deformation and flow of materials under the influence of an applied stress. The interfacial rheology of a surfactant film normally accounts for the interfacial viscosity and elasticity of the film. The interfacial viscosity can be classified with interfacial shear viscosity and interfacial dilational viscosity. Films are elastic if they resist deformation in the plane of the interface and if the surface tends to recover its natural shape when the deforming forces are removed. The interfacial elasticity can also be classified with interfacial shear elasticity and interfacial dilational elasticity (6, 7, 12). Malhotra and... 

Neustadter et al. (166) earlier measured interfacial dilation elasticities, e j, and viscosity, qj, for Iranian crude oil/water and deduced that the extent of the relaxation process was not a function of time due to the lack of change in viscosity, q j, at fixed frequency. However, as frequency changed, q j decreased, indicating that the relaxation involved the interchange of bulk material to and from the interface. The increased elas-ticy, ej, with time suggested that there was irreversible adsorption of high-molecular-weight species. 

Measurements of interfacial dilatational elasticities and viscosities at crude oil/water interfaces have been carried out. The dilatational viscosity at fixed frequency does not change materially with time over a period of - 3 hours. This strongly suggests that the nature and extent of the film relaxation process is not a function of time. The fall in dilatational viscosity with frequency indicates that the relaxation process involves mainly bulk to interface diffusion interchange. The increase of dilatational elasticity with time reflects the irreversible adsorption of slowly adsorbing high molecular weight species. 

The interfacial dilational elasticity, , results from the presence of interfacial tension gradients due to inhomogeneous surfactant or polymer films. The regions that are depleted from the film have higher interfacial tension than those containing the adsorbed film. As a result, an interfacial tension gradient djIdA is set and the Gibbs dilational elasticity may be defined as... 

Prins et al. [19] found that a mixture of sodium dodecyl sulfate (SDS) and dodecyl alcohol gives a more stable 0/W emulsion when compared to emulsions prepared using SDS alone. This enhanced stability is due to the higher interfacial dilational elasticity e for the mixture when compared to that of SDS alone. Interfacial dilational viscosity did not play a major role since the emulsions are stable at high temperature whereby the interfacial viscosity becomes lower. This correlation is not general for all surfactant films since other factors such as thinning of the film between emulsion droplets (which depends on other factors such as repulsive forces) can also play a major role. 

A study on a commonly used demulsifier, namely, a phenol-formaldehyde resin, elucidated how various parameters such as interfacial tension, interfacial shear viscosity, dynamic interfacial-tension gradient, dilatational elasticity, and demulsifier clustering affect the demulsification effectiveness [1275]. 

Our goal is to develop a property-performance relationship for different types of demulsifiers. The important interfacial properties governing water-in-oil emulsion stability are shear viscosity, dynamic tension and dilational elasticity. We have studied the relative importance of these parameters in demulsification. In this paper, some of the results of our study are presented. In particular, we have found that to be effective, a demulsifier must lower the dynamic interfacial tension gradient and its ability to do so depends on the rate of unclustering of the ethylene oxide groups at the oil-water interface. 

Since there is no change in surface tension with a change in the rate of a pure liquid surface (i.e., d A/d II = infinity), the elasticity is zero. The interfacial dilational viscosity, ks, is defined as... 

By way of introduction, consider the dilational experiment of fig. 3.49. At t = t the monolayer is instantaneously subjected to a stress r° = r = r°, which is kept constant till t = t, after which it is suddenly removed. Panels (b) and (c) refer to the strain response AA/A for a purely elastic and a purely viscous monolayer, respectively, The elastic monolayer directly follows the applied stress after cessation it relaxes instantaneously. The viscous one starts to flow after cessation of the stress the flow stops. The height of the block in panel (b) is equal to r / K (compare [3.6,18]) whereas in panel (c) the interfacial (dilational) viscosity follows from the slope as... 

In the presence of liquid flow, the situation becomes more complicated due to the creation of surface tension gradients [17]. These gradients, described by the Gibbs dilational elasticity [17], e, initiate a flow of mass along the interface in direction of a higher surface or interfacial tension (the Marangoni effect), e is given by the... 

In the book by Joos [16] as well as in original papers, some special cases of this general approach have been discussed. It was shown that such stress relaxation experiments are well suited for studying the dilational rheology of interfacial layers, which yield the dilational elasticity as a function of the effective surface age teff... 

In recent years, several theoretical and experimental attempts have been performed to develop methods based on oscillations of supported drops or bubbles. For example, Tian et al. used quadrupole shape oscillations in order to estimate the equilibrium surface tension, Gibbs elasticity, and surface dilational viscosity [203]. Pratt and Thoraval [204] used a pulsed drop rheometer for measurements of the interfacial tension relaxation process of some oil soluble surfactants. The pulsed drop rheometer is based on an instantaneous expansion of a pendant water drop formed at the tip of a capillary in oil. After perturbation an interfacial relaxation sets in. The interfacial pressure decay is followed as a function of time. The oscillating bubble system uses oscillations of a bubble formed at the tip of a capillary. The amplitudes of the bubble area and pressure oscillations are measured to determine the dilational elasticity while the frequency dependence of the phase shift yields the exchange of matter mechanism at the bubble surface [205,206]. 

Many experiments have been proposed for measuring the interfacial shear viscosity and elasticity and interfacial dilatational viscosity and elasticity at gas/liquid and liquid/ liquid interfaces [22]. Interfacial shear viscosities of different oil/aqueous systems have been studied worldwide. Some experimental results indicate that low interfacial shear viscosities do not necessarily imply that an emulsion will be unstable [23]. The dilatational rheology is based on area changes due to an expansion or compression of a fluid surface and stress relaxation experiments. The experiment results show that the interfacial dilatational properties can be much higher than the interfacial shear properties for the same system [15,24-27]. This makes researchers believe that interfacial dilatational viscosity and elasticity may have a better relationship with the stability of the emulsion than with interfacial shear properties. 

Due to the changes in the interfacial area a compression or expansion of the adsorption layers is generated which induces a relaxation process in order to re-establish its equilibrium state. By monitoring the evolution of interfacial tension with time the dilational elasticity and the relaxation mechanism can be obtained. 

Yq surface tension of the pure solvent r = r F2 total adsorption 5 relative oscillation amplitude AHj molar standard enthalpy of transfer Anp density difference Sjj dilational elasticity dilational viscosity rig shear viscosity relative area change X=k/xn dimensionless rate constant interfacial chemical potential n = Yq-y surface tension co, co2 partial molar areas in - 27if circular frequency... 

For an air/liquid system a measure of the surface-tension variation resulting from the imposed periodic area variation in the Langmuir trough is performed. If both dilational viscous = (f) and dilational elastic j = e (f) data are needed, and if a Langmuir-type trough is used, then one barrier can be oscillated and another barrier can be used to adjust the extent of the interfacial area. The calculation of the complex modulus, , requires complete scans at different frequencies. 

This is the dilational elastic modulus or the interfacial tension gradient which is in phase with the area change. 

In the case of a soluble nonionic siufactant the detected increase in a in a real process of interfacial dilatation can be a pure manifestation of siuface elasticity only if the period of dilatation,At, is much shorter than the characteristic relaxation time of surface tension x, At x (21). Otherwise, the adsorption and the surface tension would be affected by the diffusion supply of siufactant molecules from the bulk of solution toward the expanding interface. The diffusion transport tends to reduce the increase in surface tension upon dilatation, thus apparently rendering the interface less elastic and more fluid. The initial condition for the problem of adsorption kinetics involves an instantaneous (At Tjj) dilatation of the interface. This instantaneous dilatation decreases the adsorptions T and the subsiuface concentrations of the species (the subsurface... 

It was discussed quite extensively, that interfacial dynamics and rheology are key properties of liquid disperse systems, such as foams and emulsions. The stability of such systems depends for example on the dilational elasticity and viscosity, however, surely not on the elasticity modulus (Borwankar et al. 1992). Here, the interfacial rheology with its frequency dependence comes into play, and data at respective frequencies will possibly correlate with the stability behaviour. 

There are some informations about monotonous decrease of the equilibrium surface tension, dilatational elasticity, and adsorption of lysozyme for non-ionic surfactant decyl dimethyl phosphine oxide (Cj DMPO) as the concentration of surfactant increases in the mixture. However, in the case of mixtures of non-ionic surfactants with more flexible proteins like P-casein, the elasticity of the interfacial layer decreases before passing through a maximum as the concentration of surfactant increases [7], Possibly, the weaker interfacial network formed by P-casein as compared to globular proteins determines the dilatational response of the mixtures. The same picture was shown for the system P-casein mixed with dodecyl dimethyl phosphine oxide (C,2DMPO). For all studied frequencies (0.005-0.1 Hz) the elasticities for adsorption layers have a maximum about 4x10" mol/1 Cj2DMPO concentration. It was shown the obtained values are very close to those measured for the surfactant alone. Thus, in this concentration region the surfactant dominates the surface layer. In our case we have... 

For the Iranian heavy crude oil/distilled water interface it seems that the dilatational elasticity, j, is not particularly high (compared with subsequent BSA values) or when compared with very cohesive insoluble interfacial films at the liquid/air interface. However, there is a measurable phase angle (0) and hence dilatational viscosity and this confirms that relaxation processes will play an important part in the behavior of the crude oil/water interface. 

The dilatational viscosity seems high however again there is little with which to compare it. The dilatational elasticity of the interfacial film increases with film age, particularly at the longer interfacial contact times (i.e., 2-3 hours). This behavior is as would be expected from previous knowledge on the pseudostatic elasticity of these interfacial films (1). 

Monolayers seldom, if ever, show an entirely viscous behavior. They always present an elastic contribution. Interfacial dilation or compression causes a change in the interfacial tension which, after releasing the stress, relaxes with a characteristic time toward equilibrium. Thus, the interfacial tension change induced by changing the interfacial area is determined by an elastic and a viscous contribution that are likely to be additive ... 

Keywords Drop and bubble shape tensiometer, interfacial tension, interfacial relaxation, dilational elasticity, adsorption of surfactants and proteins... 

The interfacial tension response to transient and harmonic area perturbations yields the dilational rheological parameters of the interfacial layer dilational elasticity and exchange of matter function. The data interpretation with the diffusion-controlled adsorption mechanism based on various adsorption isotherms is demonstrated by a number of experiments, obtained for model surfactants and proteins and also technical surfactants. The application of the Fourier transformation is demonstrated for the analysis of harmonic area changes. The experiments shown are performed at the water/air and water/oil interface and underline the large capacity of the tensiometer. 

The fundamentals of drop and bubble shape analysis have been discussed in detail above. In the next section examples are given to demonstrate the various applications of the profile analysis tensiometer. Besides dynamic surface and interfacial tensions, results are shown for trapezoidal and sinusoidal relaxation experiments from which the dilational elasticity can be derived. The experiments selected are not only for model surfactants of high chemical purity but also for technical surfactants for which effective data can be deduced. 

The relaxation studies with transient and harmonic area perturbations allow the determination of the dilational elasticity of interfacial layers. For protein the properties of adsorption layers at the water/air interface are very much different to those obtained at the water/oil interface which can be explained by the structure of the molecules in the interfacial layer. Consequently, the dilational elasticity and relaxation behaviour is very different at these two interfaces. 

The experimental results shown in Figure 12.10 demonstrate the capacity of the drop and bubble shape technique. After the adsorption process has reached an equilibrium state, over a period of time of about 6 h, some square pulses of the drop area are subsequently, produced. Such area perturbations are suitable for determining the surface dilational elasticity of the interfacial layer. Efficient dosing systems even allow a sinusoidal area change, again providing information... 

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